Over-voltage protection device

ABSTRACT

An overvoltage protection device consists of a number of parallel connected columns containing nonlinear resistance stacks between upper and lower spark gap stacks. The spark gap stacks provide arc extension by magnetic effect. Cross-impedances are arranged between the columns for transferring ignition pulses. The spark gaps, nonlinear resistors and connecting members for the cross-impedances are so dimensioned and arranged that the impedance increase of a column after ignition in comparison with the impedance increase of one or more subsequent ignited columns is so great that the current through the first column is less than a value corresponding to its lowest current carrying voltage. The cross-impedances may be connected between the bottom of an upper spark gap stack and the top of a lower spark gap stack.

. United States Patent Inventors Erland Nilsson; 3,094,648 6/1963Nilsson 3 l 7/70X Asle Schei, Ludvika, Sweden 3,198,986 8/1965 Luehn'nget al 3 l 7/70X 33%;1968 Primary Examiner-James D. Tramrnell 1 eAttorney.lennings Baile Jr.

Patented Feb. 23, 1971 y Assignee Allmanna Svenska ElektriskaAktiebolaget Vasteras, Sweden Priority Nov. 6, 1967 fa t? ABSTRACT: Anovervoltage protection device consists of a number of parallel connectedcolumns containing nonlinear resistance stacks between upper and lowerspark gap stacks. The spark gap stacks provide arc extension by magneticef- OVER-VOLTAGE PROTECTION DEVICE 6 Claims 1 Drawing fect.Cross-impedances are arranged between the columns for transferringignition pulses. The spark gaps, nonlinear re- U.S. sistors andconnecting members for the cro55-impedances are 3 17/68, 317/69 sodimensioned and arranged that the impedance increase of a II?- Clcolumnafter ignition in comparison with the impedance in- Field of Search317/31, 61, r a f o e r more subsequent ignited columns is so great 73,74, 68, 69, 70; 315/36 that the current through the first column is lessthan a value corresponding to its lowest current carrying voltage. TheReferences and cross-impedances may be connected between the bottom ofUNITED STATES PATENTS an upper spark gap stack and the top of a lowerspark gap 2,989,664 6/l96l Zoller 317/70 stack.

i 2 3 4 g 6e 2 ilt E le e i 8 a 5 a 5 a 7 E .L 7 E 7 7 /O 1 [6T 1 IOIT E0'] E PATENTED FEB23I97| 3,566,197

INVENTOR. EQLAN D N I LSSON A6 LE SC HEl OVER-VOLTAGE PROTECTION DEVICEBACKGROUND OF THE INVENTION 1. Field of the Invention The inventionrelates to an over-voltage protection device for alternating current andhigh power.

2. The Prior Art For overvoltage protection devices for alternatingcurrent and high power several parallel columns can be used containingspark gaps and nonlinear overvoltage resistors in order to be able toabsorb the energy during a discharge interval. Since all the columnscannot be made exactly the same, at least one of them will spark overbefore the rest and, if no special precautions are taken, the column orcolumns sparking over first would be forced to take care of the entiredischarge, whereas other columns would not take part at all. Thus a gooddeal of the effect intended with parallel-connection is lost. Theproblem has been solved by connecting cross-impedances between thevarious columns so that the alternation in voltage distribution within acertain rod which occurs during the discharge interval through onecross-impedance influences another column so that this ignites and allthe columns therefore take part in the discharging.

With alternating current the protection device is normally extinguishedwhen the voltage passes through zero the first time after the ignitionand no extra steps need be taken to extinguish the overvoltageprotection device.

With direct current the situation is quite different. If an overvoltageoccurs in a direct current network so that an overvoltage protectiondevice sparks over, the overvoltage will not necessarily drop as quicklyas with alternating current since there is no zero-passage for thevoltage. There is thus a considerable risk that the overvoltageprotection device will burn for such a long time that it is destroyeddue to overheating.

SUMMARY OF THE INVENTION The present invention relates to an overvoltageprotection device which is particularly suitable for use in directcurrent networks but which can also be used in alternating currentnetworks.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawing shows aschematic diagram for an overvoltage protection device according to theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS According to the invention theovervoltage protection device consists of a number of parallel-connectedcolumns 1, 2, 3, 4, and so on between the network and earth. The drawingshows the use of four columns, but both larger and smaller numbers arepossible and the number depends, amongst other things, on the value ofthe overcurrents arising and also on the type of the separate elements.A rod is built up of a centrally arranged nonlinear overvoltageresistance stack 5 consisting of a required number of resistance blocksand upper and lower spark gap stacks 6 and 7, respectively. Between twopreferably adjacent columns is a cross-impedance 8 which is suitablyresistive and which may be nonlinear like the overvoltage resistor. Inthe embodiment shown in the drawing a cross-impedance is connectedbetween the overvoltage resistance stack at the upper end in one columnand the lower end of the overvoltage resistance stack in the othercolumn. When the overvoltage protection device is not in operation thesetwo points are electrically equal but geometrically different. It is,however, possible and in certain cases suitable to connect thecross-impedance in a different manner, for example between twogeometrically equal points.

The mutual control between the discharge process in two adjacent rods isnot only determined by the impedance element 3 but also by the impedanceof the spark gap stack itself and the control can be further influencedwith the help of shunt impedances 9 and 10 inserted parallel to the rodsand also connected to thecross-impedance 8. The shunt impedancespreferably consist of capacitive elements, but other impedance elementsare also feasible.

As mentioned previously, it is not necessary for all the parallel rodsto be alike. On the contrary, it may even be suitable to make themdifferent so that at least one of them ignites earlier when anovervoltage occurs. If the column 1 in the drawing is so dimensionedthat it ignites first, the voltage drop over the upper and lower sparkgap stack in this column will fall to a low value before the extensionof the are due to the magnetic field has started to make itselfnoticeable. Most of the voltage drop in the column 1 then lies in thebeginning across the resistance stack 5 and this voltage drop istransferred to the lower spark gap stack of the column 2 with a timedelay determined by the cross-impedance 8 and the impedance of the sparkgap itself, the shunt impedance 9 and 10. This increased voltage ignitesthe lower spark gap 7 stack of the rod 2 and then also the upper sparkgap 6 so that the whole column 2 sparks over and in turn ignites thecolumn 3 in the same way as the column 1 ignited the column 2.

After a certain time the arc is extended in the spark gap stacks in thecolumn 1 and the impedance of this column increases so much that itgives a voltage drop across the column which corresponds to the voltagedrop over the other rods. The current in the column 1 is thus forceddown to such a low value that the are through the column is extinguishedand the current through the column is interrupted. At the moment ofextinction the voltages over the upper and lower spark gap stacks in thecolumn 1 are practically equal, but the voltage over the lower spark gapstack in the column 2 is very small since the arc extension has not hadtime to become apparent. This means that the voltage across thenonlinear resistance stack in the column 2 increases the voltage overthe upper spark gap stack 6 in the column I so that, after a certaintime, this reignites and the entire column 1 reignites and takes part inthe discharge. A similar process takes place in each of the other rodsand in this way the discharge voltage can be maintained at the desiredlevel and a high extinction voltage is ensured, since the arc in thespark gap is extinguished as soon as it has been blown out to its fulllength due to the influence of the magnetic field and no appreciableheating of the gap takes place. Thus a repeated ignition and extinctionof the arc in all the rods is obtained so that the current is constantlymoved from one column to the other. This means that the dischargecurrent can pass the overvoltage protection device without itsextinguishing capacity being reduced.

By suitably dimensioning the cross-impedances 8 and possibly also theshunt impedances, the column 3 can be brought to spark over before thecolumn I is extinguished. In this way it can be arranged that two rodsburn simultaneously if this is suitable in order to obtain asufficiently powerful discharge.

The energy absorption capacity of the protection device is substantiallydetermined by the number of parallel-connected legs and can in principlebe made any size.

lclaim:

l. Overvoltage protection device comprised of a plurality ofparallel-connected columns containing nonlinear resistors and spark gapspromoting arc extension by magnetic effect, and cross-impedancesarranged between the columns for transferring ignition pulses betweenthe columns in which the spark gaps, nonlinear resistors and connectingmembers for the cross-impedances are so dimensioned and arranged withinthe different columns that the impedance increase of a column afterignition in comparison with the impedance increase of one or moresubsequently ignited columns is so great that the current through thefirst mentioned column is less than a value corresponding to the lowestcurrent carrying voltage of this column.

2. Overvoltage protection device according to claim 1, in

I which the ignition delay between the columns is determined by thecross-impedances together with the impedance of the spark gaps.

dimensioned that the rate of increase speed of their are volt- 7 age ishigher than in the other columns.

6. Overvoltage protection device according to claim 1, in which thecross-impedance is connected geometrically and also electrically whennot in operation, to the same point on the spark gap in each stack.

1. Overvoltage protection device comprised of a plurality ofparallel-connected columns containing nonlinear resistors and spark gapspromoting arc extension by magnetic effect, and crossimpedances arrangedbetween the columns for transferring ignition pulses between the columnsin which the spark gaps, nonlinear resistors and connecting members forthe cross-impedances are so dimensioned and arranged within thedifferent columns that the impedance increase of a column after ignitionin comparison with the impedance increase of one or more subsequentlyignited columns is so great that the current through the first mentionedcolumn is less than a value corresponding to the lowest current carryingvoltage of this column.
 2. Overvoltage protection device according toclaim 1, in which the ignition delay between the columns is determinedby the cross-impedances together with the impedance of the spark gaps.3. Overvoltage protection device according to claim 2, having shuntimpedances comprising capacitive elements.
 4. Overvoltage protectiondevice according to claim 1, in which in at least one of the columns thespark gaps are so dimensioned that their arc voltage is higher than inthe other columns.
 5. Overvoltage protection device according to claim1, in which in at least one of the columns the spark gaps are sodimensioned that the rate of increase speed of their arc voltage ishigher than in the other columns.
 6. Overvoltage protection deviceaccording to claim 1, in which the cross-impedance is connectedgeometrically and also electrically when not in operation, to the samepoint on the spark gap in each stack.